NZ aftershock was more damaging than main shock

When it comes to earthquake damages, magnitude is not everything.

The February 22, 2011, 6.3-magnitude earthquake that struck the South Island of New Zealand was less powerful  than the  September 3, 2010, 7.1 quake that struck to the west of Christchurch. Black circles on the diagram below from NASA (the source of this information) represent earthquakes in the region between these dates. Red circles show the locations of the magnitude 6.3 quake and aftershocks on February 22 and the morning of February 23. Larger circles represent stronger earthquakes. Yellow shows urban areas, including Christchurch.

In fact, the USGS characterized the February 22 event as an aftershock of the quake on September 3.

The Darfield earthquake in September 2010 caused no casualties, even though it had a higher magnitude. Besides striking closer to a major population center, the 6.3-magnitude Christchurch earthquake had a depth of just 5 kilometers (3 miles). The New Zealand Herald reported that, whereas the Darfield quake happened in the early morning hours, the February 22 quake struck at the “worst possible time” of day—at the lunch hour when city streets were crowded with shoppers, diners, office workers, and school children. Moreover, some of the buildings that collapsed may have been weakened by the September 2010 quake.

Seismogram on New Zealand earthquake

I have been learning to use the software and database Global Earthquake Explorer for my geophysics course this semester. It seems very useful, although I am still trying to figure out how I can recover ground motion amplitudes in order to have my students calculate magnitudes.

Below is the seismogram from the Christchurch, New Zealand, earthquake, as recorded by the seismograph at South Karori, New Zealand, and retrieved using GEE.

Conflicing research on state of stress in Chile

The New York Times carries an article today about the state of stress in the vicinity of the epicenter of last February's magnitude 8.8 Chile earthquake. Different sources of information  - seismic waves, gps, tsunami waves - have yielded somewhat different pictures. The epicenter occurred outside of the 200-mile-long Darwin gap which had not seen a large quake since one that Charles Darwin observed in 1835.  Did the occurrence of the 2010 quake lead to a stress increase in this gap, and a higher risk of an upcoming large quake?  We have the usual conclusion: more data are needed.

200th anniversary of the New Madrid earthquakes

From the New Madrid bicentennial site:

"Several of the largest historical earthquakes to strike the continental United States occurred in the winter of 1811-1812 along the New Madrid Seismic Zone, which stretches from just west of Memphis, Tennessee into southern Illinois. These earthquakes produced at least three temblors between magnitude 7-8, and hundreds of aftershocks. 

"In 2011 and 2012, there will be events held throughout the central United States observing the 200th Anniversary of the great 1811 and 1812 New Madrid earthquakes that forever changed the mid-western landscape. These quakes were felt across the U.S. and as far south as the Gulf of Mexico, and as far north as Canada. Organizations from across the United States will participate in the bicentennial events, which range from conferences, workshops, public outreach events, multi-state earthquake exercises, field trips, and more.  Organizers of the New Madrid Bicentennial events (NMB) are already planning the following activities:

Bicentennial Kickoff - February 11, 2011
Earthquake Awareness Month - February 2011
Seismological Society of America Meeting - April 2011
Great Central U.S. ShakeOut - April 28, 2011 @ 10:15a.m.
National Level Exercise (NLE2011) - May 16-20, 2011
Earthquake Awareness Month - February 2012
National Earthquake Conference - April 2012
EERI Annual Conference - April 2012
St. Jude Dream Home Partnership - April 2012"